Powertrain for a hybrid vehicle with all-wheel drive capability and method for controlling wheel slip

ABSTRACT

A hybrid-electric powertrain and control method for a vehicle having forward traction wheels and rearward traction wheels in an all-wheel drive configuration. An engine and an electric motor deliver power through delivery paths to the traction wheels. The power delivery paths may have multiple ratio gearing so that more power can be delivered mechanically to improve powertrain performance and to allow the electric motor size to be reduced. The powertrain may include a controller for automatically balancing power distribution to the forward and rearward traction wheels to avoid wheel slip.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/276,773, filed Mar. 14, 2006, now U.S. Pat. No. 7,314,424 issued Jan.1, 2008, which is a divisional of U.S. application Ser. No. 10/747,429,filed Dec. 29, 2003, now U.S. Pat. No. 7,163,480, which is acontinuation-in-part of U.S. application Ser. No. 10/463,046, filed Jun.17, 2003, entitled “Method and Apparatus for Transferring Torque and aHybrid Vehicle Incorporating the Method and Apparatus,” now abandoned,which is a continuation of U.S. application Ser. No. 09/848,038, filedMay 3, 2001, now abandoned. Applicants claim priority to thoseapplications and to U.S. provisional application Ser. No. 60/447,081,filed Feb. 14, 2003. U.S. Pat. No. 7,128,677 issued on Oct. 31, 2006 isa divisional application of Ser. No. 10/747,427 filed Dec. 29, 2003, nowU.S. Pat. No. 7,086,977 which is a continuation-in-part of U.S.application Ser. No. 10/463,046 filed Jun. 17, 2003. All of thesepatents and applications are assigned to the assignee of the presentapplication.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present application relates to powertrains for hybrid-electric,all-wheel drive vehicles and to a method for managing power distributionto vehicle traction wheels.

2. Background Art

U.S. patent application Ser. No. 10/463,046, filed Jun. 17, 2003,identified above, and U.S. Pat. No. 5,856,709 disclose hybrid-electricpowertrains capable of delivering driving torque to traction wheels ofan automotive vehicle through a geared transmission that establishesmultiple powerflow paths from an engine power source and an electricalpower source. The '046 patent application is assigned to the assignee ofthe present invention.

The powertrains disclosed in the '709 patent, as well as the copending'046 patent application, may be adapted to both front-wheel drive andrear-wheel drive configurations for hybrid-electric vehicles. U.S. Pat.No. 6,176,808 discloses another example of a hybrid-electric vehiclepowertrain of this type.

The '046 patent application and the '808 and '709 patents areincorporated in the disclosure of this application by reference.

In known geared transmission configurations for hybrid-electric vehicleswith multiple power sources, an electric motor typically is connected tothe driving wheels through a set of fixed ratio gears. This providesimproved launch torque as motor torque is multiplied by the gearing. Ahigh torque multiplication for the torque flow path for the motor,however, requires a compromise between the maximum output speed requiredand the initial acceleration torque needed during a vehicle launch ifthe electric motor gearing has a fixed ratio. The need for this is duein part to the use of fixed ratio gearing in the driving torque flowpaths from the multiple power sources to the traction wheels.

In the case of a hybrid-electric vehicle powertrain of the kinddisclosed in the previously identified copending patent application,recovery of regenerative electrical energy in the powertrain may belimited because the electric motor is connected mechanically only to therear traction wheels.

SUMMARY OF THE INVENTION

The present invention is adapted particularly for use in an all-wheeldrive hybrid-electric vehicle powertrain. To balance the differentrequirements for improved performance and improved fuel economy, adownstream torque multiplying gear set is used, which avoids an increasein the motor and generator sizes. In this fashion, more power istransmitted mechanically.

The present invention avoids the need for a compromise betweenperformance and fuel economy by providing powerflow paths from thetraction motor and the engine to the traction wheels with multiple gearratios. This provides independent control over the launch torque and themaximum vehicle speed. Further, the present invention improves theability of the powertrain to recover regenerative electrical energy byinstalling the electric motor in the powertrain in coaxial dispositionwith respect to the vehicle front wheels and by connecting electricallythe electric motor to the battery.

In embodiments of the present invention included in this disclosure, theengine is connected to the carrier of a planetary gear unit. Like thefront-wheel drive embodiment of the powertrain disclosed in thepreviously identified copending patent application, the generator of thepresent invention is connected to the sun gear and the ring gear isconnected to the traction wheels through a two-speed gearing arrangementand a differential-and-axle assembly. The traction motor is coaxiallydisposed on the front-wheel axis, the motor rotor being directlyconnected to the axle shafts for the forward traction wheels. In analternate embodiment of the present invention, a second planetary gearset is placed on the front-wheel axis between the motor generator and afront traction wheel.

Unlike the powertrain configurations of the copending patent applicationwhere the motor is connected through gearing to the ring gear of aplanetary gear unit at the torque output side of the motor, the motor inthe powertrain of the present invention is not connected directly to thering gear. However, a mechanical powerflow path between the motor andthe ring gear is maintained as the rear traction wheels drive the fronttraction wheels in a powerflow path through the road.

The traction motor is directly coupled to the road, as explained above,and there is a shift available in the powertrain configuration of thepresent invention. This shift is designed to occur during low loadoperating modes for the transmission. The motor at the front wheels,during the shift, can adequately fill any loss of driving torque as thetransmission is shifted from one ratio to the other. This eliminates aso-called “torque hole” or torque interruption during a synchronousshift.

In one of the embodiments disclosed in this application, there are twogear sets, each having two ratios. The ratios for the two gear sets arestaggered, as are the shift points. A positive power delivery to thewheels thus is maintained. Because of this characteristic, a synchronousshift is not required in either of the gear sets. Control of the shiftthus is simplified.

As previously indicated, the multiple powerflow paths in the all-wheeldrive hybrid-electric powertrain of the invention cause torque to betransmitted electrically as the rear wheels drive the front wheels andengine power and motor power are delivered to different sets of wheels.Torque compensation during engine start-up could, under somecircumstances, be difficult to achieve. To improve torque compensationand improve torque compliance at each set of wheels, it is possible withan alternate embodiment of the present invention to slip the low ratioand high ratio clutches of the transmissions at a fixed torque that isequivalent to the reaction torque required for an engine start-up. Ifslip is detected during engine start-up, the pressure at the clutches isincreased to increase the clutch engagement torque. When the controllerdetects that the engine is at a stable combustion speed, the enginetorque is increased to a desired level while slipping the clutch. Assoon as the desired engine speed is achieved, the clutch is fullyengaged, thus creating a torque transfer to the wheels.

In still another embodiment of the invention, improved performanceduring engine start-up can be achieved by using an additional reactionbrake that grounds the ring gear on the torque output side of thegenerator motor while the drive clutches are disengaged. This eliminatesa torque transfer to the wheels during engine start and engine shutdown.Thus, no torque disturbances are transferred to the wheels. Once theengine speed is brought up to a stable operating level, the drive clutchis engaged, thereby again transferring power to the traction wheels.

Another aspect of the present invention comprises a strategy formodifying the distribution of torque to the two sets of traction wheelsfor an all-wheel drive configuration of a hybrid-electric vehiclepowertrain. In such powertrains, power can be transmitted between thefront and rear traction wheels in any proportion. This improves fueleconomy.

During operation in the all-wheel drive mode, driver expectation is tohave a specific percentage power distribution to the front wheels and tothe rear wheels. This is achieved by making the powertrain operate in apositive power distribution mode, whereby mechanical energy isdistributed to the rear wheels and electrical energy is distributed tothe front wheels. The engine speed and torque are continuously modifiedin this configuration to achieve the correct balance between the twoenergy sinks.

Slip control can be implemented when slip is detected, or slip controlcan be used continuously. It is possible using this embodiment of theinvention for an all-wheel drive operating mode to be selected, whereinpower is distributed between the front traction wheels and the reartraction wheels with a distribution ratio of around 50% to 50% as thecontroller alters engine speed and torque to maintain the desired powerdistribution to the front and rear traction wheels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an all-wheel drivehybrid-electric vehicle powertrain having two coaxially disposedplanetary gear units with a generator and an engine on a common axis andan electric motor on the front wheel axis;

FIG. 1 a is a schematic representation of a powertrain of the kinddisclosed in the copending patent application previously identified,which can be adapted for use with a front-wheel drive hybrid-electricvehicle powertrain;

FIG. 2 is a schematic gearing arrangement for an all-wheel drivehybrid-electric powertrain configuration wherein power is transmittedelectrically between the sun gear of a first planetary gear unit and anelectric motor mounted on a front wheel axis;

FIG. 3 is a schematic representation of another embodiment of theinvention wherein the reaction element of a second gear unit on theengine generator axis may be grounded by a friction brake and atwo-speed gear unit connects the motor to the front wheels;

FIG. 4 is a schematic diagram of another embodiment of the inventionwherein two planetary gear units are situated on an engine axis in amanner similar to the arrangement of FIG. 2, wherein a secondpressure-operated friction brake, which may be a slipping brake, is usedto reduce force on the rear wheels;

FIG. 4 a is a schematic representation of a powertrain similar to thepowertrain of FIG. 4, but an additional friction brake is provided toground the ring gear of the first planetary gear unit while the driveclutch and brake for the gear units are disengaged;

FIG. 4 b is a schematic representation of a multiple-ratio,hybrid-electric powertrain similar to the powertrain of FIG. 4 a,although the multiple ratio transmission between the torque outputelement of the planetary gearing comprises a countershaft geararrangement rather than a planetary gear arrangement;

FIG. 5 is a flow chart illustrating software control strategy for anall-wheel drive transmission operating in either a four-wheel drive modeor a two-wheel drive mode; and

FIG. 6 is a schematic block diagram showing the control strategy for anall-wheel drive hybrid-electric powertrain that compensates for tractionwheel slip at the front wheels and for traction wheel slip at the rearwheels, wherein the engine speed and torque are continuously modified toachieve a desired torque balance.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 is a schematic illustration of a hybrid-electric vehiclepowertrain with all-wheel drive capability. For the purpose ofdescribing the mode of operation and the performance of the powertrainillustrated in FIG. 1, reference first will be made to the front-wheeldrive configuration shown in FIG. 1 a. This front-wheel driveconfiguration is disclosed in the previously identified copending patentapplication.

As seen in FIG. 1 a, an engine 10 is connected to transmission inputshaft 12 through a mechanical spring damper 14. Shaft 12 is connected tothe carrier 16 of a planetary gear unit 18. The sun gear 20 of the gearunit 18 is connected to the rotor 22 of an electric generator 24. Anoverrunning coupling or brake 26 prevents the carrier 16 and the enginefrom being driven with reverse motion while allowing the generator todeliver torque to the wheels when the engine is turned off.

The ring gear 28 of planetary gear unit 24 is connected drivably tocountershaft drive gear 30 and to countershaft gear 32, thus driving theintermediate shaft 34. An electric traction motor 36 is drivablyconnected to the intermediate shaft through gears 38 and 32.Countershaft gear 40 meshes with the ring gear of adifferential-and-axle assembly 42 for the traction wheels. Atransmission oil pump 44 is drivably geared to shaft 12. Battery 37 iselectrically coupled to motor 36 and generator 24.

When a vehicle with the transmission arrangement shown in FIG. 1 a is ina highway cruise mode, the generator brake 46 is applied. Thisestablishes a geared connection between the engine driven shaft 12 andthe differential-and-axle assembly 42.

If the generator brake 46 is applied, the powerflow path is fullymechanical. The power source can be fully electrical if the vehicle islaunched from a standing start with the engine off. A positivedistribution of power occurs when the generator absorbs torque and themotor is applying drive torque. When the motor absorbs torque and thegenerator rotates and contributes power that assists the engine, anegative power distribution occurs. With both positive powerdistribution and negative power distribution, a part of the energy istransferred electrically and part is transferred mechanically.

In FIG. 1, a first embodiment of a hybrid-electric vehicle powertrain ofthe invention is illustrated. It includes front traction wheels 48 andrear traction wheels 50. A high-torque induction motor 52 is mountedcoaxially with respect to the axis of axle shaft 54, the rotor of themotor 52 being connected directly to the wheels. Other types of electricmotors could be used if that would be feasible.

An internal combustion engine 56 drives a torque input shaft 58 for afirst planetary gear unit 60. The connection between the gear unit 60and the engine 56 includes a mechanical damper 62.

The planetary gear unit 60 comprises a ring gear 64, a planetary carrier66, and a sun gear 68, the carrier being connected directly to thetorque input shaft 58. An overrunning coupling 70 provides a torquereaction for the carrier 66 as carrier torque is delivered to thetransmission case.

The ring gear 64 is connected to sun gear 72 of a second planetary gearunit 74 coaxially disposed with respect to the gear unit 60. Aconnection between ring gear 64 and sun gear 72 is established by torquetransfer shaft 76.

Ring gear 78 of planetary gear unit 74 can be braked by friction brake80 to provide a torque reaction point for planetary gear unit 74 astorque is delivered from shaft 76 to the carrier 82, which in turn isdrivably connected through torque output shaft 84 todifferential-and-axle assembly 86 for the rear traction wheels 50. Thecarrier 82 can be connected selectively to shaft 84 through frictionclutch 88. When clutch 88 is applied, the speed ratio across planetarygear unit 74 is 1:1. When brake 80 is applied and clutch 88 is released,ring gear 78 acts as a reaction element. Clutch 88 and brake 80 define aclutch and brake friction element sub-assembly.

A generator 90 has a rotor connected directly through a sleeve shaft tothe sun gear 68 of gear unit 60. Sun gear 68 can be braked by brakingthe rotor of the generator 90 by means of friction brake 94.

As mentioned before, engine torque is delivered to the carrier 66. Powerthen is distributed through two powerflow paths by the planetary gearunit 60, the reaction torque on gear 68 being distributed to the rotorof the generator 90. When the generator speed is greater than zero, itgenerates power for use by the motor 52. When the generator speed isless than zero or negative, it acts as a motor as torque is distributedto the sun gear 68 through the shaft 92 from the rotor of the generator90. This is a so-called negative power distribution as the generatoracts as a motor. If the generator speed is positive or greater thanzero, there is a positive power distribution. The generator, as it actsin either of the two operating modes, is able to control engine speed,and thus it can be considered to be an engine speed controller.Controlling the engine in this fashion is more efficient thanconventional methods for controlling the engine using air flow sensors,fuel sensors, or spark advance and retard devices in the case of aninternal combustion engine with spark ignition.

The generator can be used to adjust engine speed so that the engine willoperate at its most efficient operating point on the engine speed-torquecharacteristic plot.

In the case of the design of FIG. 1 a, when the powertrain is actingwith negative power distribution, the generator acts as a motor. Thegenerator needs to supply negative torque to control the engine speed asthe motor acts as a generator. The speed of the generator also isnegative, so the ring gear 64 is driven in a positive direction. Theeffective torque acting on the ring gear then is the algebraic sum ofthe torque provided by the motor and the torque provided by the engine.

In the embodiment of FIG. 2, a first planetary gear unit 96 is included.Its function corresponds to the function of planetary gear unit 60 inthe embodiment of FIG. 1. Gear unit 96 includes a ring gear 98, acarrier 100, and a sun gear 102. Ring gear 98 is connected to ring gear104 of a second planetary gear unit 106 located between the gear unit 96and a differential-and-axle assembly 108 for rear wheels 110.

Engine 112 drives carrier 100 through a mechanical torque flow pathprovided by a damper 114 and driveshaft 116. Sun gear 102 and the rotorof a generator 118 are braked by a friction brake 120 against atransmission housing.

The planetary gear unit at the torque output side of the gear unit 96includes a sun gear 122, which can be anchored selectively by frictionbrake 124. When the brake 124 is released, a friction clutch 126 can beused to connect selectively sun gear 122 to ring gear 104, therebyestablishing a gear ratio of unity in the gear unit 106. When the clutch126 is released and the sun gear 122 is anchored by the brake 124,torque ratio is increased, thereby multiplying the output shaft torquein shaft 128. Clutch 126 and brake 124 define a clutch and brakefriction element sub-assembly.

As in the case of the embodiment of FIG. 1, an electric motor 130 ismounted coaxially on front wheel axle shaft 132 for driving front wheels134.

In the embodiment of FIG. 3, the elements of the powertrain between theengine and the differential-and-axle assembly are similar to elements ofthe powertrain of FIG. 1. For this reason, the numerals used in FIG. 3correspond to the numerals used in FIG. 1 to illustrate correspondingelements, although prime notations are added to the numerals used inFIG. 3.

In the embodiment of FIG. 3, a motor 136 is mounted on and is drivablyconnected to axle shaft 138, which drives the front traction wheels 48.

Shaft 138 is connected to the carrier 144 for gear unit 142. Ring gear146 can be braked by a selectively engageable friction brake 148, or itcan be clutched to the carrier by means of a selectively engageablefriction clutch 150. This provides a two-speed ratio capability for thefront wheel torque flow path.

Sun gear 152 is drivably connected by sleeve shaft 154 to the rotor forthe electric motor 136.

As in the case of the design of FIG. 2, the design of FIG. 3 offers asolution to the problem of the inherent inefficiency of regenerativebraking using gearing arrangements with a conventional so-called“north-south” configuration, as distinct from a front-wheel drivetransaxle configuration. In the case of FIG. 3, the traction motor isinstalled on the front-wheel drive axis, as in the design of FIG. 2, butit is connected to the front traction wheels through a two-speedplanetary gear unit. Thus, there are two two-speed transmissions in thegearing arrangement of FIG. 3. The ratios of the gearing arrangement ofFIG. 3 are staggered, which eliminates problems associated withsynchronous shifts in the case of a ratio change with a single planetarygear unit where a ratio change requires disengagement of one frictionelement while synchronously engaging a companion friction element.

In the powertrain configuration of FIG. 4, the gearing arrangement issimilar to the gearing arrangement of FIG. 2 except that in the case ofFIG. 4 a slipping brake 160 is used to provide a torque reaction for sungear 122′. In the embodiment of FIG. 4, the elements of the powertrainthat have a corresponding element in the powertrain of FIG. 2 have beenidentified by the same reference numerals used in FIG. 2, although primenotations are added to the numerals in FIG. 4.

In the case of the powertrain illustrated in FIG. 4, during engine stopand start events, the brake 160 is slipped at a fixed torque equivalentto the reaction torque required to achieve engine starting. Brake 160will permit the ring gear 98′ to act as a reaction member for the firstplanetary gear unit 96′. As the generator rotor drives sun gear 102′,the generator acts as a starter motor to develop engine cranking torque.Clutch 126′ and brake 160 define a clutch and brake friction elementsub-assembly.

In the embodiment illustrated in FIG. 4 a, the elements of theconfiguration are the same as the elements of the embodiment of FIG. 4except for the addition of a “ring gear-to-ground” brake 162, whichprovides a direct torque reaction for the ring gear 98″ during enginestarting as the generator rotor drives the sun gear to achieve enginecranking torque. At this time, the clutch and the brake for thedownstream planetary gear unit 106″ can be released.

The elements of the configuration of FIG. 4 a that have counterpartelements in the configuration of FIG. 4 are assigned the same referencenumerals, although double prime notations are used. In the case of theembodiment of FIG. 4 a, the brake 160′ is used as a low-speed reactionbrake, while the clutch 126′ acts as a direct-drive clutch. Brake 160′and the clutch 126″ provide two speed ratios through the downstreamgearing 106″. With the brake 160′ and the clutch 126″ disengaged, torquetransfer to the wheels is completely eliminated during engine start andengine shutdown. Once the engine is started and is brought up to astable operating speed, the friction elements can be engaged to transferpower to the wheels.

In the design of FIG. 4 b, the gearing configuration is similar to theconfiguration of FIG. 2. The downstream planetary gear unit 106 of FIG.2, however, is replaced in the design of FIG. 4 b by countershaftgearing having two ratios. The countershaft gearing comprises a firstgear 164 connected to shaft 128″ and a countershaft gear 166, whichmeshes with gear 164. A first clutch 168 is selectively engageable toestablish and disestablish a driving connection between gear 164 andring gear 98″ of the planetary gear unit 96″.

In the embodiment of FIG. 4 b, triple prime notations are used with thenumerals to designate elements that have a counterpart in the embodimentof FIG. 2.

Countershaft gear 166 can be connected selectively to a small pitchdiameter countershaft gear 169 by a selectively engageable clutch 170.The countershaft gearing of FIG. 4 b thus provides a high ratio and alow ratio corresponding to the high ratio and low ratio established bythe planetary gearing of FIG. 2 at 106. The mode of operation and theperformance of the gearing configuration of FIG. 4 b is the same as themode of operation and the performance of the gearing arrangement of FIG.2. One design may be preferred over the other, however, depending uponpackaging requirements of the powertrain assembly in a hybrid-electricvehicle.

FIGS. 5 and 6 illustrate in schematic form the control strategy for anall-wheel drive hybrid-electric powertrain. As previously described,power can be distributed between the front and rear wheels to achievethe best fuel economy. The power distribution between the front and rearwheels, as explained previously, includes a torque transfer that occurselectrically between the motor and the first planetary gear unit 60 inFIG. 1, for example.

When the hybrid-electric vehicle powertrain is in an all-wheel drivemode, the expectation of the driver is to have a specific distributionof power to the front wheels and to the rear wheels. This can beachieved using the strategy of FIGS. 5 and 6 to force the vehicle toenter a so-called positive power distribution mode, whereby mechanicalenergy is distributed to the rear wheels and electrical energy isdistributed to the front wheels. The engine speed and torque arecontinuously modified to achieve the correct balance between the twopower flow paths.

FIG. 6 is a strategy that is implemented if one of the traction wheelsat the rear of the vehicle or at the front exhibits slip. The slip isdetected by a powertrain controller and a slip control, illustratedschematically in FIG. 6.

When the vehicle is operating in a pure electric drive mode, only thefront wheels are driven, the source of the driving torque being themotor. When the vehicle is operating in positive power distributionmode, the engine develops power and that power is converted by thegenerator to electrical energy. The reaction torque on the ring gear forthe first planetary gear unit (i.e., gear unit 60 of FIG. 1) isdelivered to the rear wheels through the downstream planetary gear unit74. Electrical energy is converted back to mechanical energy by thetraction motor, which is electrically coupled to the generator. Thatmechanical energy is distributed to the front wheels to provideadditional traction. The percentage of torque distribution to the frontwheels and to the rear wheels can be varied between 0% and 100% for eachwheel set.

If the controller conditions the powertrain for operation in a parallelmode, a generator brake, shown at 94 in the case of FIG. 1, is applied.All the energy developed by the engine then is transferred to the rearwheels mechanically. There is no electric power distribution.

If the controller conditions the powertrain to operate with negativepower distribution, the motor generates energy as the front wheels aredriven, and that energy is transferred to the generator. The generatorrotor is driven in a reverse direction and the engine speed then isreduced. In this instance, more than 100% of the total energy istransferred to the rear wheels, and the front wheels actually recoverenergy. If the operator selects all-wheel drive operation, a powerdistribution ratio of approximately 50% to 50% requires the vehicle tobe in a positive power distribution mode. The controller will alterengine speed and torque to achieve the optimum power distribution. Theoperating mode is selected based upon the best fuel economy point in anengine speed-torque characteristic plot.

In normal drive at low power demand, there may be a negativedistribution of power, whereas when the power demand is high, a positivepower distribution mode is used. As the motor at the front wheelsgenerates energy, it acts as a generator. The motor is electricallycoupled to the generator, which causes torque to be distributed to thetraction wheels. During high power demand, the generator develops powerfor the motor, so both the front wheels and the rear wheels providedriving torque.

If the operator causes the powertrain to assume an all-wheel drive mode,the engine speed must be increased to prevent the generator rotor frommoving in a negative direction. With the increased engine speed,however, the optimum operating point that the controller will use toachieve maximum fuel economy with two-wheel drive will no longer be theoperating point during four-wheel drive, so there will be less enginetorque available for transferring to the rear wheels. On the other hand,at this time, the generator is driven faster, so electrical power isdelivered to the front wheels.

FIG. 5 is a flow chart for a control algorithm. At control block 166,the controller will determine whether four-wheel drive operation hasbeen requested. If the four-wheel drive mode is not on, the engine speedis determined by a first table in the electronic memory of thecontroller as indicated at action block 168. The engine speed iscontrolled by the generator, as previously explained. The generatorspeed can be decreased to make the engine operate at its best operatingpoint.

The generator has better torque control than an engine since, typicallyin an engine control system, torque of the engine is determined by airflow, or fuel rate or engine spark timing adjustments, which do notproduce a torque response as fast as a torque response resulting fromgenerator speed change.

At action block 168, the correct engine speed for each of the possibleoperating modes is obtained from a table that is compatible with theelectric drive mode, the parallel drive mode, the positive powerdistribution mode, or the negative power distribution mode.

The input for the determination at action block 168 includes a driverdemand input 170, which takes into account power losses due to the powerdemands of accessories such as an air conditioning compressor, a waterpump, etc. Vehicle speed, which is an actual vehicle speed measurement,is another input, as shown at 172.

The torque of the engine is determined, as shown at action block 174,using the power demand and the engine speed information. The enginetorque then is delivered to the transaxle, as previously explained andas schematically illustrated at 176 in FIG. 5.

If the four-wheel drive mode is selected, as shown at action block 178,a different engine torque and speed table is used, as shown at actionblock 180 in FIG. 5. The table used in action block 180 is differentthan the table used in action block 168 because, as previouslyexplained, the generator, which develops electrical energy used by themotor for the front wheels, causes the engine to operate more slowly.The system is calibrated so that high engine efficiency will bemaintained even though it is not the optimum efficiency available fortwo-wheel drive, as represented by the table of action block 168.

As in the case of action block 174, the torque is determined at 182 forall-wheel drive operation using the engine speed and driver demandinformation for all-wheel drive.

FIG. 6 shows a control strategy for compensating for wheel slip of thefront wheels or wheel slip of the rear wheels. Each wheel, as is wellknown in the design of automatic brake control systems, contains a wheelspeed sensor. Wheel speed information from a wheel speed sensor can beused to detect slip at the front wheels as well as at the rear wheels.

The controller will receive driver power demand information at 184 andvehicle speed information at 186 for the determination of engine speed,torque of the engine and torque of the motor, as shown at action block188. For any of the modes that may be selected, including the electricmode, the parallel mode, and the positive and negative powerdistribution modes, the information developed at action block 188 iscombined at action block 190 with front wheel slip information fromaction block 192. At action block 190, a change in engine speed, or adelta engine speed, is subtracted from the actual engine speed. Thedelta engine speed is achieved by decreasing generator speed, aspreviously explained. A change in engine speed will result in a changein engine torque received from action block 188. The torque informationfor a given change in engine speed is obtained from the table containedat action block 188. The motor torque at the front wheels is decreasedby a delta motor torque. Because of the reduction in generator speedwith the torque at the front wheels decreasing and the torque at therear wheels increasing, the total wheel torque remains the same.

If rear wheel slip is detected at action block 194, the engine speedwill be increased by a delta engine speed value, as shown at actionblock 196. The torque on the engine that accompanies the increase inengine speed will decrease by a delta torque value, as indicated ataction block 196. Further, the torque on the motor will be equal to thewheel torque plus a delta motor torque, as the generator speed isaltered by the controller. As in the case of action block 190, the totalwheel torque remains the same, as shown at action block 196.

Each action block 190 and 196 produces the system controller outputparameters, which are indicated at action block 198. Those parametersare used in the calculation of a delta engine torque, the delta enginespeed and the delta motor torque.

When engine speed is increased at action block 196, the operating pointon the engine speed and torque characteristic plot deviates from theoptimum point, so less torque is transferred to the rear wheels, but thegenerator is driven faster. Because of this, more power goes to thefront wheels. In the case of action block 190, the converse of thissequence is true.

Although embodiments of the invention have been disclosed, it will beapparent to those persons skilled in the art that modifications may bemade without departing from the scope of the invention. All suchmodifications and equivalents thereof are intended to be covered by thefollowing claims.

1. A powertrain for an all-wheel drive vehicle with forward and rearwardsets of traction wheels, the powertrain comprising: an engine; anelectric generator; an electric motor; a geared transmission having afirst torque delivery element connected drivably to the engine, a secondtorque delivery element connected drivably to the generator, and atorque output element; the electric motor being drivably connected tothe forward set of traction wheels and being electrically coupled to thegenerator; and a multiple ratio gear unit drivably connecting the torqueoutput element of the geared transmission to the rearward set oftraction wheels, whereby mechanical energy is distributed to therearward set of traction wheels and electrical energy is distributed tothe forward set of traction wheels; the multiple ratio gear unitcomprising discrete ratio gearing with distinct ratio steps in a torqueflow path from the torque output element of the geared transmission tothe rearward traction wheels.
 2. The powertrain set forth in claim 1wherein the geared transmission comprises: a planetary gear unit havinga ring gear, a planetary carrier and a sun gear; the engine beingdrivably connected to the carrier; the generator being drivablyconnected to the sun gear; and the torque output element of the gearedtransmission being drivably connected to the ring gear.
 3. Thepowertrain set forth in claim 2 wherein the geared transmission furthercomprises: a generator brake, the generator brake being applied toeffect operation of the powertrain in a parallel mode, whereby all ofthe energy for powering the vehicle is transferred to the rearward setof wheels mechanically.
 4. The powertrain set forth in claim 1 whereinthe engine develops mechanical energy and the geared transmissiontransfers the mechanical energy to the generator, whereby the generatorconverts the mechanical energy to electrical energy for the electricmotor in a positive power distribution operating mode.
 5. The powertrainset forth in claim 1 wherein the electric motor functions to generateelectrical energy as it is driven by the forward set of traction wheels,whereby the generator develops power opposing engine power to reduceengine speed in a negative power distribution mode.